Convection is the main mechanism of thermal energy transfer in the atmosphere. On a global basis, the non-uniform temperature distribution on the surface of the earth causes convection cycles that result in the prevailing winds. If the earth were not rotating, a huge convection cell would be established. The equator is the hottest portion of the earth because it gets the maximum radiation from the sun. Hot air at the equator expands and rises into the atmosphere. Cooler air at the surface flows toward the equator to replace the rising air. Colder air at the poles travels toward the equator. Air aloft over the poles descends to replace the air at the surface that just moved toward the equator. The initial rising air at the equator flows towards the pole, completing the convection cycle. The net result of the cycle is to bring hot air at the surface of the equator, aloft, then north to the poles, returning cold air at the polar surface back to the equator.
The simplest picture of convection on the surface of the earth is not quite correct, because the effect produced by the rotating earth, called the Coriolis effect, has been neglected. The Coriolis effect is caused by the rotation of the earth and can best be described by an example.
If a projectile, aimed at New York, were fired from the North Pole, its path through space would be in a fixed vertical plane that has the North Pole as the starting point of the trajectory and New York as the ending point at the moment that the projectile is fired. However, be the time that the projectile arrived at the end point of its trajectory, New York would no longer be there, because while the projectile was in motion, the earth was rotating, and New York will have rotated away from the initial position it was in when the projectile was fired. A person fixed to the rotating earth would see the projectile veer away to the right of its initial path, and would assume that a force was acting on the projectile toward the right of its trajectory. This fictitious force is called the Coriolis force and this seemingly strange behavior occurs because the rotating earth is not an inertial coordinate system.
The Coriolis effect can be applied to the global circulation of air in the atmosphere, causing winds in the northern hemisphere to be deflected to the right of their original path. The global convection cycle described above still occurs, but instead of one huge convection cell, there are three smaller ones. The winds from the North Pole flowing south at the surface of the earth are deflected to the right of their path and become the polar easterlies. As the air aloft at the equator flows north it is deflected to the right of its path and eventually flows in a easterly direction at approximately 30o north latitude. The piling up of air at this latitude causes the air aloft to sink to the surface where it emerges from a semi permanent high-pressure area called subtropical high. The air at the surface that flows north from this high-pressure area is deflected to the right of its path producing the mid-latitude westerlies. The air at the surface that flows south from this high-pressure area is also deflected to the right of its path and produces the northeast trade winds. Thus, it is the non uniform temperature distribution on the surface of the earth that is responsible fro the global winds.
Transfer of thermal energy by convection is also very important in the process called the sea breeze. Water has a higher specific heat than land and fro the same radiation from the sun, the temperature of the water does not rise as high as the temperature of the land. Therefore, the land mass becomes hotter than the neighboring water. The hot air over the land rises and a cool breeze blows off the ocean to replace the rising hot air. Air aloft descends to replace this cooler air and complete the cycle. The net result of the process is to replace hot air over the land surface by cool air from the sea. This is one of the reasons why so many people flock to the ocean beaches during the hot summer months. The process reverses at night when the land cools faster than the water. The air then flows from the land to the sea and this is called land breeze.
This same process of thermal energy transfer takes place on a smaller scale in any room in your home or office. Let us assume there is a radiator situated at one wall of the room. The air in contact with the heater is warmed, and then rises. Cooler air moves in to replace the rising air and a convection cycle is started. The net result of the cycle is to transfer thermal energy from the heater to the rest of the room. All these cases are examples of what is called natural convection.
To help the transfer of thermal energy by convection, fans can be used to blow the hot air into the room. Such a hot air heating system is called a forced convention system. A metal plate is heated to a high temperature in the furnace. A fan blows air over the hot metal plate, then through some ducts, to a low-level vent in the room to be heated. The hot air emerges from the vent and rises into the room. A cold air return duct is located near the floor on the other side of the room, returning cool air to the furnace to start the convection cycle over again. The final result of the process is the transfer of thermal energy from the hot furnace to the cool room.
